1. Field of the Invention
The present invention relates to improving the oxidation stability of lubricants, especially hydrocarbon based lubricating oils, by adding thereto a combination of a secondary diarylamine and a substituted para-phenylenediamine.
2. Description of Related Art
Lubricants, such as those used in a variety of machinery, are susceptible to oxidative deterioration during storage, transportation, and usage, particularly when such lubricants are exposed to high temperatures and iron catalytic environments, which greatly promote their oxidation. This oxidation, if not controlled, contributes to the formation of corrosive acidic products, sludge, varnishes, resins, and other oil-insoluble products, and may lead to a loss of designated physical and tribological properties of the lubricants. It is therefore a common practice to include an antioxidant in lubricants to prevent, at least to some extent, oxidation, so as to extend their useful life. Lubricant compositions containing various secondary diarylamines as antioxidants are widely known in the art. The use of para-phenylenediamines is also known, although to a lesser extent.
U.S. Pat. No. 2,451,642 discloses para-phenylenediamines as useful antioxidants for lubricating oil compositions for use in environments where iron-catalyzed oxidation reaction can take place.
U.S. Pat. No. 2,718,501 discloses a stabilizer system consisting of an aromatic amine with at least two aromatic rings, including para-phenylenediamine, and an organic aliphatic sulfur compound, which is said to be suitable for stabilizing mineral hydrocarbon lubricating oils, synthetic hydrocarbon oils, and polyalkylene glycol oils.
U.S. Pat. No. 5,232,614 discloses substituted para-phenylene diamines that are said to be effective antioxidants capable of protecting crankcase lubricating oils from thickening and sludge formation after prolonged exposure to oxygen at elevated temperature.
WO 94/22988 discloses a fuel composition said to improve the antiwear and viscosity controlling properties of an internal combustion engine lubricating oil during operation of the engine. Small amounts of the fuel composition combine with the engine lubricating oil during engine operation and this provides an antioxidant boost to the lubricating oil. Preferably the fuel contains at least 57 g/1000 liters of a substituted dicyclic aromatic amine which is free of benzylic hydrogen atoms such as a mono- and/or di-α-methyl styrene alkylated phenylenediamine and/or a hindered phenol such as a monostyrenated mono-isobutenated cresol or di C16 alkyl phenol. A synergistic effect is said to be demonstrated by a mixture of the aromatic amine and hindered phenol.
IN 151,316 discloses the use of N,N′-di-sec-butyl-p-phenylenediamine as antioxidant for a solvent extracted, dewaxed and hydrofinished mineral oil based lubricating oil composition for heavy duty air compressors.
JP 53,051,206 discloses N,N′-2-naphthyl-p-phenylenediamine as an antioxidant to improve the oxidation stabilities of ester or mineral oil based lubricating oils that also contain disulfides.
JP 59,020,392 discloses a lubricant composition comprising N,N′-di-sec-butyl-p-phenylenediamine for pressure forming of oil tanks. The lubricant composition also contains hindered phenolic antioxidant.
Polish PL 149,256 discloses the use of phenyl-naphthyl-para-phenylene diamine for polyalkylene glycol based fire resistant hydraulic fluids.
Soviet Union Patent 1,155,615 discloses a mineral oil and cephalins based lubricant composition comprising diphenyl-para-phenylenediamine for cold deformation of metals.
The foregoing disclosures are incorporated herein by reference in their entirety.
Heretofore, there has been no recognition of the use of secondary diarylamines in a particular combination with substituted para-phenylenediamine that provides unexpected synergistic results in stabilizing lubricants, e.g., lubricating oils, especially those intended for use in a demanding environment, such as where high temperature and metal catalyzed oxidative reactions may take place.